1. Field of the Invention
This invention pertains to the general field of testing of light-emitting materials. In particular, it pertains to a method and related apparatus for predicting the electroluminescent properties of individual light emitting diodes (LEDs) from measurements conducted on the semiconductor waver from which the LEDs are produced.
2. Description of the Prior Art
The characterization of light-emitting semiconductor structures at the wafer-level (i.e., after forming the p-n junction and the active quantum well layers, but prior to the chip processing steps) is typically carried out with a non-destructive wafer probe. Parameters such as the current-voltage curve, the diode ideality factor, its reverse saturation current and its spectral properties at the device level are of critical importance for the characterization of the LEDs manufactured from wafers. To that end, a conductive probe is temporarily placed in contact with the top of the epi-wafer (p-GaN) layer while another electrode contacts the n-GaN layer through either the edge of the wafer or through other means that allow access to the n-GaN layer. Such typical layout is illustrated in FIG. 1. When energized, the conductive probe, the semiconductor p-n junction structure on the wafer and the electrode form a temporary light-emitting device. By injecting a known current into the junction, light will emit and the spectral properties and their relationship with electrical properties can be measured and characterized.
Thus, one of the objectives of using probes to characterize light-emitting structures on a semiconductor wafer is to predict both the optical and the electrical properties at the device level after the wafer has been processed to produce LEDs. However, due to factors such as the difference in geometry and in the electrical contact methods and configurations, the parameters measured by a probe on wafer will normally differ from those measured on a device produced from it. For example, at the device level the electrodes are deposited permanently on device layers, as illustrated in FIG. 2, thereby producing a well defined current pass-channel. At the wafer level, where the probe is only mechanically pressed with a predetermined loading force against the surface of the wafer, the roughness of both probe and wafer surfaces produce a contact resistance that is not present in the individual device where the p-electrode and n-electrodes are formed onto respective device layers. Also, the distance between the two electrodes, which is fixed for device measurements, changes from location to location during wafer measurements with a corresponding change in the current channel position and length, as can be understood from FIG. 3.
Therefore, there remains a need for a wafer testing method that enables the reliable prediction of the optical and electrical characteristics of the LEDs produced from the wafer. This invention describes an approach that has shown reliable and repeatable results to that end.